Understanding the connection between normally evolving galaxies and those with extreme nuclear activity, the so-called Active Galactic Nuclei, is a fundamental problem in the study of both galaxy evolution and of AGN. From HST images of the 96 BL Lacertae objects observed in our Cycle 5 and 6 programs, we have determined the absolute magnitudes and the radial profiles of over half of the host galaxies, taking advantage of HST's excellent resolution to constrain the profile shapes very close to the centre. This is a complete sample, chosen from complete x-ray, radio, and optical samples, and spanning a redshift range of 0 to 1. Our preliminary results agree with current unified schemes models - the BL Lacs do tend to reside in giant elliptical galaxies, and morphological anomalies are common, suggesting recent interactions in many of our sample. Our preliminary results also suggest a correlation between nuclear and host galaxy magnitude, a relationship which has been tentatively claimed for other classes of AGN, but not yet confirmed. Our current program is to analyse the HST images jointly with deep ground based data. By combining the superior resolution of HST with the low surface brightness sensitivity of large ground based telescopes, we are able to constrain the galaxy morphologies much more tightly. We present here the current results of this analysis.